A logarithmic sensor that operates a photodiode in an open circuit similarly to a solar cell and measures an output voltage, has been known (refer to Non-Patent Document 1). This uses a relationship in which a potential difference, namely, a voltage occurring when a current flows in the forward direction of a PN junction, is proportional to the logarithm of the current. The current in the forward direction is replaced with a photocurrent generated by photoelectric conversion and the voltage in the forward direction of the PN junction is monitored, so that a signal including the photocurrent compressed logarithmically is acquired.
In addition, a solid-state image pickup device using the logarithmic sensor in the solar-cell mode, is also publicly known. A combination of the logarithmic sensor in the solar-cell mode and a typical storage-type CMOS image sensor by space division, has been proposed and a combination of the two by time division has been proposed. The reason why the combinations have been proposed, is that the logarithmic sensor in the solar-cell mode has dark and low-illuminance characteristics degraded.
Applying the structure of the logarithmic sensor in the solar-cell mode, to the structure of the typical storage-type CMOS image sensor corresponds to a structure having a photodiode (PD) contacted directly. In this case, the dark and low-illuminance characteristics degrade due to factors: kTC noise cannot be removed and an afterimage occurs since the PD cannot be depleted completely; white spots and a dark current attributable to an interface state degrade since pinning cannot be performed to a PD surface; and the like. These occur in an illuminance range before output enters a logarithmic region since the PN junction capacitance of the logarithmic sensor cannot be sufficiently charged.
For example, even in a case where the logarithmic sensor in the solar-cell mode and the typical storage-type CMOS image sensor are combined, the dark and low-illuminance characteristics of the logarithmic sensor in the solar-cell mode do not necessarily improve. Needless to say, the single logarithmic sensor has a problem remaining, and even for the combination, there is a need to ensure the transition period between a linear signal and a logarithmic output, so that the low-illuminance characteristic of the logarithmic sensor is required. In a case where a sufficient transition period is not provided, noise is noticeable in signal switching and thus there is a risk that image quality degrades.
In addition, the logarithmic sensor in the solar-cell mode is a sensor that monitors a photocurrent amount to output a signal, and thus no element to retain the signal is provided and no global shutter function has been achieved.